Add the butter and oil to a large saute pan over medium high heat. Add the onions to the pan and saute until lightly browned, about 20 to 25 minutes, stirring occasionally. Add the wine to the pan and reduce for 2 to 3 minutes.
Add the beef base, parsley and black pepper to the pan and stir. Remove the pan from the heat and let cool.
Preheat the oven to 425degrees F.
Slice the baguette into 1/4-inch-thick pieces and place onto a sheet pan. Spread each slice of bread with the onion mixture and top with 1 tablespoon of the Swiss cheese.
Place the sheet pan directly into the oven and bake for 5 minutes or until the cheese is melted and bubbly.
Carefully remove the sheet pan from the oven and place the bruschetta onto a serving platter. Serve immediately.
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